In current harddisk and optical disk applications partial response maximum likelihood detectors (PRML, Viterbi) are used for recovering the data from a channel data stream. FIG. 1 shows high frequency channel data representing a data stream from an exemplary optical channel signal. The channel data are not distorted by asymmetry. In this case a data slicing process is easily able to distinguish between two different signal states (high and low). However, the data patterns recorded on a recording medium may be distorted by an asymmetric data writing procedure. For example, on optical recording media one reason for asymmetry is Blooming, which is caused by non-linear effects during data recording. FIG. 2 shows an example of a channel data stream deteriorated by strong signal asymmetry. Due to the asymmetry a simple slicing algorithm is not able to properly distinguish between data bits representing digital high or low, since adaptive equalization means fail. This problem becomes worse when noise is present. In order to properly recover the channel data from this signal a compensation scheme is needed.
Usually a digitised optical channel data stream is resynchronized to the channel clock using a phase locked loop (PLL) and a sample rate converter (SRC), then equalized with a filter setting representing the inverse channel behaviour and finally detected using a simple slicer and a bit detector.
An adaptive approach using a maximum likelihood decoder calculates the difference of the recovered data to the equalized data, e.g. with the least mean square (LMS) method, and uses this as adjusting information in order to get a coefficient set for the equalizer, which better fits to the channel characteristic. The LMS updating is typically implemented as follows:Coeffnew=Coeffold+μ×ε(filtered vitout,eqout)×channel data
However, some approaches only consider the direction (+ or −) of the error signal ε.
It has been found that the cause of signal asymmetry shows different behaviour during a transition from one storage state (e.g. land) to another storage state (e.g. pit). By separately compensating both transition defects a better signal data representation is found for decoding the contained data bits using a maximum likelihood approach. FIG. 3 shows an advanced version of the above scheme, which makes use of this finding by adjusting a separate equalizer for each edge of the incoming data signal.
It is an object of the invention to improve the methods known from prior art.